This invention relates to an improved linear variable displacement transducer (LVDT) or rotational variable displacement transducer (RVDT) and improved circuitry for the operation thereof.
LVDTs and RVDTs are used to generate an electrical output proportional to the mechanical displacement of a moveable core generally of ferromagnetic material. The electrical output provides an electrical measurement of distances and displacements which in turn may represent, for example: force, torque, pressure, velocity, or acceleration. As such, LVDTs and RVDTs are in common use in a number of well known configurations. The basic elements include a moveable core, a primary coil and a pair of secondary coils which are so arranged as to have the moveable core magnetically link the primary coil with the secondary coils.
The advantages associated with the use of differential transformers over other displacement transducers, such as the resistance potentiometer, are the absence of contacts, infinite resolution, low and near constant output impedance, and input-output isolation. The disadvantage is that operation has heretofore been limited to excitation by conventional alternating voltages, with the accompanying drawbacks of residuals, transients, harmonic generation, and shielding problems.
Two recent patents representative of the state of the art for conventional AC excitation of the primary coil within an LVDT are U.S. Pat. No. 4,514,689 issued to William A. Gerard and U.S. Pat. No. 4,450,443 issued to Charles R. Dolland. The 4,514,689 patent detects the peak value and polarity of each secondary coil output voltage and provides an output voltage representative thereof. The 4,450,443 patent converts the output of the secondary coil from a sinusoidal (AC) waveform to a square-wave signal prior to sampling a value proportional to the peak value of the output waveforms. Both feature high resolution and accuracy, however, both also retain some of the inherent limitations associated with the use of an AC excitation signal.